1. Field of the Invention
The invention generally relates to a device and method for routing (e.g., dividing and subdividing) bands of optical signals and, more particularly, to a multiport, tunable periodic filter based on polarization interferometry, and a method for tuning the transmission peaks and frequency spacing of the optical signals.
2. Description of Related Art
The demand for increased data transmission capability continues to grow. Users of DWDM systems are pressing for greater bandwidth utilization with 50 GHz and tighter channel spacing, adding further challenge to upgrade existing DWDM""s struggling with 100 GHz channel spacing.
Several approaches for providing building blocks for all-optical networks capable of meeting the challenging demands of service providers and users involve tunable filters. These include (but are not limited to), e.g., cascaded Fabry-Perot (resonant cavity) and Mach-Zehnder (interferometry) components for squeezing more and more channels into a free spectral range or limited bandwidth. Such devices and examples of their utilization are described in Green, Fiber Optic NETWORKS, Prentice Hall, cha. 4, (1993). Disadvantages associated with these devices include, e.g., long response time or slow tuning speed, poor crosstalk performance and device complexity and fabrication tolerance. Other lattice and Mach-Zehnder component filter designs used as band splitters are also described in EP 0 724 173A1, U.S. Pat. No. 5,680,490 and OFC ""98 Technical Digest, paper ThQ7 by Nolan et al. These devices lack optimum contrast ratio and desired tuning capability.
The inventors have therefore recognized a need for a DWDM filter device that caters to the immediate and future requirements for high speed network systems without the disadvantages associated with current components and approaches. Accordingly, the invention describes a periodic filter device that has the attributes of accurate and easy tuning, polarization independence, high contrast ratio over the whole 1.5 xcexcm telecommunications band, environmental stability, conformity with the ITU grid, and others, that will be apparent from the description, drawings and claims which follow. Applications of the invention include, but are not limited to, band splitters, wavelength monitors and wavelength selective cross-connect components.
Accordingly, the invention is broadly directed to a device for routing optical signals. Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the apparatus and method particularly pointed out in the written description and claims hereof as well as the appended drawings.
An embodiment of the invention is directed to a tunable optical channel routing device including an input for a plurality of optical channels having a frequency period; first means for separating a polarization state of the input optical channels into orthogonal (s and p) polarization states; means for temporally retarding one of the orthogonal polarization states with respect to the other orthogonal polarization state for producing an ordinary beam and an extraordinary beam for each polarization state; second means for separating the polarization states of an output from the retarding means again into orthogonal (s and p) polarization states; and, an output for the plurality of optical channels from the second means wherein each channel of a first group of output channels has a center frequency and the first group of output channels has a frequency period that is different from the frequency period of the input channels, and wherein each channel of a second group of output channels has a center frequency and the second group of output channels has a frequency period different from the frequency period of the input channels and is interleaved with the first group. The device provides a contrast ratioxe2x89xa720 dB over the 1.5 xcexcm spectral band, which is essentially limited by the dispersion of the retarding component (e.g., birefringent material).
In different aspects of this embodiment, the optical components for separating the orthogonal polarizations can be polarizing beam splitters (PBS""s) or polarizing beam displacers (PBD""s), and the components for providing the ordinary and extraordinary beams can be a birefringent crystal or a birefringent optical fiber. While the period of the groups of output channels is tailored by the thickness or amount of birefringent material traversed by the light, fine tuning is achieved by rotating the birefringent crystal about its c-axis, changing the length of the birefringent fiber, or through the use of a phase compensator such as a liquid crystal. An optical path length compensator such as, e.g., a half-wave plate or a selective index transparent material is used as necessary to maintain equal optical path lengths to minimize polarization mode dispersion (PMD). 1xc3x972 and cascaded 2xc3x972 devices according to embodiments of the invention provide further application flexibility as, e.g., optical cross-connect components.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.